This invention applies to diffraction networks frequently used in optical applications particularly for light coupling within a guide. The periodic function of the network enables it to generate several orders, diffracted in different directions, from a single incident wave; this property can be used to phase-match waves propagated outside the guide and those propagating in the guide modes. When the network is produced by periodically varying the index, the efficiency with which the light energy in the incident wave is coupled to the diffraction orders depends directly on the amplitude of the variation. It is known that the efficiency of the network can be modified by dynamically varying the refractive index of the material used. To achieve this, one solution employed in the prior art is to use the electrooptical effect to electrically vary the network index, the periodic function of the index to be modified then being determined by rows of interleaved electrodes which create, within a material capable of generating electrooptical effects and with an optical index n, zones with index n+.DELTA.n and zones with index n-.DELTA. n using electrodes held at potential +V and electrodes held at potential -V.
FIG. 1 illustrates this type of structure in which it is possible to generate, by applying field .xi., zones with index n+.DELTA. n separated from zones with index n-.DELTA. n by zones with index n.
Typically, this can be a wave guide produced by known techniques within a lithium niobate crystal whose optical axis c is perpendicular to the plane of the guide. If it is assumed that the variation in index .DELTA. n is solely due to the field applied by the interleaved electrodes, which is colinear with axis c, the network of optical indices shown in FIG. 1 is obtained.
However, the major drawback to such a network of optical indices lies in the complexity of the electrodes required, which creates problems due to peak effects between electrodes, these effects leading to high voltages and the destruction of the guide or the electrodes themselves. Moreover, a complex electrode structure is an impediment to using the network at very high frequencies.